Double superheterodyne receivers, which are also known as double conversion receivers or triple detector receivers, are commonly used in, for example, UHF and RF communications. Such receivers provide high gain without instability, good suppression of the image frequencies, and high adjacent channel selectivity.
In these receivers, an incoming RF carrier frequency is mixed with a first local oscillator signal to produce a first IF signal. The first IF signal is then mixed with a second local oscillator signal to provide a second, or final IF signal, and such a signal is provided to a detector to detect the information carried on the second IF carrier.
In the transmitter/receiver used in cellular phones these days, the first locally generated oscillator frequency is also used to help generate the transmit frequency. The transmit frequency is generated by a transmitted frequency generator using the first locally generated oscillator frequency and a transmit offset frequency. The transmit offset frequency is produced by a transmit offset loop. Previous systems were DAMPS systems which used a phase detector reference of 19.44 MHz at the phase detector. In these DAMPS systems, a Gilbert cell mixer is used as a phase detector, which is an analog-type mixer. Other systems use frequency phase type detectors as phase detectors. In order to meet cellular standards, the system must be able to come up and be on frequency and transmitting within a predetermined period of time after the system is turned on. In addition, in order to conserve current in the radio, the intermediate frequency of the transmit offset loop must come up within 100 microseconds of being turned on. In order to fulfill these criteria, the phase detector frequency in the transmit offset loop must be very high. In previous DAMPS systems, the reference frequency was injected into the transmit offset loop and allowed the system to meet the lock time criteria set forth in the cellular standards.
However, problems occur in new dual-band communication systems which can be switched between different frequency bands. For example, a system may be capable of operating in the 800 MHz range for Damps systems and in the 1900 MHz range for PCS systems. In PCS systems, the frequency 19.44 MHz is a very important frequency for the correct operation of the digital signal processors within the system. Thus, the 19.44 MHz reference frequency must be maintained. However, when dual-band systems switch between different frequency bands, the 19.44 MHz frequency does not allow the system to generate a transmit offset phase detector reference frequency that is high enough to allow the system a sufficient lock time. To overcome this problem, some systems provide for fractional-N division but this adds great complexity and thus cost to the system.
Thus, there is a need for a method and apparatus for generating transmit and receive channel frequencies using a common reference crystal frequency wherein the system has a sufficiently high phase detector frequency in the transmit offset loop so as to meet the criteria set forth in the cellular standard.